The present invention relates generally to a method of preventing the accumulation of hydrogen gas in enclosed spaces at elevated temperatures and particularly to the use of organic compounds, combined with catalysts, as hydrogen getters at elevated temperatures.
There are numerous industrial operations that require the movement of high temperature fluids from one location to another such as heat pipes, thermosyphons or the tubulars used in oil fields, petrochemical plants, air separation plants and solar collectors to transport heated gases or liquids over long distances. Because of the elevated temperatures as well as other physical constraints inherent in these operations metal tubes or pipes must be used. As a consequence of the outgassing or corrosion of these metal components and thermal decomposition of the fluids transported therein, hydrogen may be formed and accumulate with deleterious effect.
In the case of tubulars and other insulated devices, in order to maintain the temperature of the fluid being transported the annular space between the coaxial inner and outer tubes can be filled with an insulating material. These insulating materials may be air or other gases or preferably insulating fibers, powders, foams or radiative heat shields. Significant improvements in thermal efficiency can be achieved by evacuating the annular space. However, the accumulation of corrosion products, particularly high conductivity gases, in the annulus causes the thermal insulating properties of these insulated tubulars to degrade over time.
Hydrogen, produced either by corrosion reactions between the fluid or gases, or constituents thereof, flowing in the pipes or tubes or diffusing in from the outside, is, because of its very high thermal conductivity, a particular problem. The degree to which hydrogen accumulation in an insulated tubular can degrade thermal insulating performance is well known. It has been estimated that partial pressures of hydrogen above 0.01 torr begin to degrade thermal insulating efficiency. The thermal conductivity of oil field tubulars utilizing an inert gas in the insulating annulus has been known to increase by a factor of from 5-8 times over the initial values in as little as a year of service due to accumulation of hydrogen. J. Roni et al. in "Insulated Tubular in Steam Injection", Kawasaki Steel Technical Report, No. 16, June 1987, pp. 74-76 show that the thermal conductivity of a vacuum insulated tubular can increase by a factor of 50 if contaminated with hydrogen.
A heat pipe works on the principle of a reflux boiler and is extremely efficient in terms of transferring large thermal fluxes. In its conventional form, the heat pipe is a closed tube in which a vaporizable fluid transfers heat from an evaporation zone to a condensation zone. Particular care is taken in the design and selection of materials of construction of heat pipes to prevent the formation of non-condensable gases in the pipe interior. Non-condensable gases, such as hydrogen, can inactivate a significant portion of a heat pipe and reducing and/or eliminating their formation from heat pipes has long been known to be of critical importance. Although hydrogen gas formation can be prevented by the proper selection of compatible containment and fluid materials, economic considerations often dictate the use of low cost materials such as carbon steel and water which generate hydrogen more rapidly.
The operating life of a lamps, either incandescent or pressured discharge lamps, can be greatly affected by the presence of certain gases in the internal lamp atmosphere. Water vapor is particularly harmful because even trace amounts can cause the evaporation and redeposition in cooler parts of the lamp of various metallic components by a process known as the "water cycle". In an incandescent lamp, for example, the temperature of the tungsten coil is sufficient to decompose water vapor into hydrogen and oxygen. The resulting oxygen reacts with tungsten in the coil to form volatile oxides which migrate to cooler parts of the lamp and condense, principally on the glass envelope. These oxide deposits are reduced by hydrogen to yield black metallic tungsten and reformed water, allowing the cycle to repeat. Removing the hydrogen formed from thermal decomposition of water vapor inside lamps by means of a hydrogen getter will prolong the useful life of the lamp. In some lamp applications, Zr-Al getters are used to remove hydrogen, however, these materials require a temperature in the range of 300-400C. in order to operate efficiently.
It has long been known that hydrogen absorbing materials, known as getters, can be used to counteract hydrogen accumulation. The use of conventional hydrogen getter materials in insulated tubulars containing low thermal conductivity gases to prolong the insulating properties of insulated tubulars has been described by Perkins in U.S. Pat. No. 3,763,935. Allen et al. discuss the use of hydrogen getters for vacuum insulated tubulars in U.S. Pat. No. 3,720,267. Ayers et al. discuss the use of active metals such as zirconium or titanium, and alloys thereof, for maintaining a vacuum in the annular space in tubulars used to inject steam into an oil well in U.S. Pat. No. 4,512,721. These metals are capable of maintaining low hydrogen partial pressures but have the disadvantage of requiring high temperatures for initial activation and/or ongoing operation because of the necessity to diffuse surface contaminants into the bulk metal thereby providing a fresh surface for continued hydrogen absorption.
Another means for removing hydrogen involves reacting the hydrogen with oxygen to form water, in the presence of a noble metal catalyst such as palladium, and trapping the water on a water absorbing material such as a molecular sieve. Labaton, in U.S. Pat. No. 4,886,048, describes the use of palladium membranes to selectively remove hydrogen from vacuum insulation jackets such as those used in evacuated solar energy collectors where the source of hydrogen is the thermal decomposition of a heat transfer fluid and where a high temperature oxidizing atmosphere is available.
The conventional hydrogen getters described in the above-referenced patents are expensive, may require special operating conditions such as high temperature regimes or ancillary reactants in order to maintain low hydrogen partial pressures, generally will not work well or at all in the presence of water and/or oxygen and may pose significant safety hazards, including fire and explosion if handled improperly, for example exposure to air. Although many hydrogen getter materials have been described and used in the past, particularly for insulated tubulars, this invention discloses a new material for removing hydrogen which has significant advantages over existing getter materials.
It is well known in the art that unsaturated carbon-carbon bonds can be reduced by hydrogen in the presence of an appropriate catalyst to form an alkane (see, for example, Fieset, L. F. and Fieser, M., Textbook Of Organic Chemistry, D. C. Heath & Co. 1950, pp. 66-69 and 86). This reaction makes possible the hydrogen getters of the present invention. In these getter systems an organic compound containing an unsaturated carbon-carbon bond, preferably an acetylenic compound, is mixed with a hydrogenation catalyst, typically a metal selected from group VIII of the Periodic Table, preferably palladium, platinum or rhodium, although other catalysts are possible, ibid. When exposed to hydrogen, the organic reactant compound is irreversibly converted to its hydrogenated analog with the aid of the associated catalyst, consequently the reaction can be carried out in a vacuum and is unaffected by the presence of normal atmospheric gases or water.